1. Field of the Invention
The present invention relates to an infrared sensor and, more particularly, to a pyroelectric infrared sensor to be used in, for example, a burglar alarm or security system for the purpose of detecting a person.
2. Description of the Related Art
FIG. 11 is a circuit diagram showing the basic circuitry of an infrared sensor. The infrared sensor generally denoted by 1 has a pyroelectric element 2. The pyroelectric element 2 has, for example, a pyroelectric substrate and electrodes formed on both surfaces of the pyroelectric substrate. An example is described below in connection with FIG. 12. Thus, a pair of pyroelectric bodies polarized in opposite directions as indicated by arrows in FIG. 11 are connected in series to form the pyroelectric element. A resistor or equivalent resistance is connected in parallel with the series connection of the pyroelectric elements. The pyroelectric element 2 has one terminal connected to the gate G of a field effect transistor (FET) 3 and the other terminal connected to the ground GND. The drain D and the source S of the FET 3 serve as the input and output terminals of the infrared sensor 1.
In the operation of this infrared sensor 1, the pyroelectric element 2 supplied with thermal energy generates pyroelectric current that is output as a voltage through an impedance transformation effected by the resistor and the FET 3. The infrared sensor 1 is sensitive even to a very low intensity of infrared rays. The high sensitivity to small energy input, on the other hand, poses a problem in that the infrared sensor 1 is not stable against external noise. In particular, the infrared sensor 1 is liable to be affected by RF noise of frequencies ranging from 100 MHz to 2 GHz, resulting in malfunction. In order to eliminate such RF noise, Japanese Laid-Open Patent Publication No. 60-125530 discloses a circuit in which capacitors are connected between the drain D of the FET 3 and the ground GND and between the source S and the ground GND of the FET 3.
In general, an infrared sensor of the kind described can have a structure as illustrated in FIG. 12. The infrared sensor 1 has a stem 4 that serves also as a case. The stem 4 has a disk-shaped metallic base 4a and three terminals 4b, 4c and 4d extending from the metallic base 4a. The terminal 4d of these three terminals 4b, 4c, 4d is electrically connected to the metallic base 4a , while other terminals 4b, 4c are insulated from the metallic base 4a. These terminals 4b, 4c, 4d are formed so as to project upward above the metallic base 4a.
A substrate 5 rests on the metallic base 4a. Pattern electrodes 6a, 6b, 6c and 6d are formed on the upper surface of the substrate 5. Holes are formed in the portions of the substrate 5 where the pattern electrodes 6a, 6b, 6c are formed, and the aforesaid terminals 4a , 4b, 4c are received in these holes. The grounding terminal 4d is connected to the pattern electrode 6a, while the electrodes 4b and 4c are respectively connected to the pattern electrodes 6b and 6c.
To the pattern electrode 6d is connected the gate G of the FET 3, while the pattern electrodes 6b, 6c are connected to the drain D and the source S of the FET 3. Capacitors 7 are connected, respectively, between the pattern electrodes 6a and 6b and between the pattern electrodes 6a and 6c. A pyroelectric element 2 is connected to the pattern electrodes 6a and 6d, through supports 8 made of an electrically conductive material. The described structure is capped with a cap 9 that has an infrared-transmissive filter.
In this infrared sensor 1, RF noise is removed by the capacitors 7 connected between the drain D of the FET 3 and the ground GND and between the source S of the FET 3 and the ground GND, whereby malfunctioning attributable to the RF noise is suppressed.
However, resistance and inductance are generated in the pattern electrodes and terminals on the substrate. Similarly, resistance and inductance are formed also in the grounding terminal, between the metallic base and the substrate. Consequently, the infrared sensor 1 has a circuit as shown in FIG. 13 that fails to stably remove RF noise due to resistance and inductance generated in the pattern electrodes and the terminals.
It is also to be noted that reduction in size of the substrate is not easy because of the necessity of providing the capacitors on the substrate. Moreover, the use of the capacitors raises the cost. For these reasons, the anti-noise measure employing capacitors can be used only for certain types of products that tolerate high price.
For the foregoing reasons, there is a need for an inexpensive infrared sensor that can stably remove RF noise and that is easy to miniaturize.